Linear compressor for a domestic appliance and domestic refrigeration appliance

ABSTRACT

A linear compressor for a domestic appliance has a piston that can move in the longitudinal direction of a cylinder between top and bottom dead center positions. A piston outer surface can be contactlessly supported against a corresponding cylinder inner surface owing to the formation of a gas thrust bearing in the radial direction of the cylinder. The piston outer surface and the cylinder inner surface each have a martensitic structure in specified regions. There is also provided a domestic refrigeration appliance with a refrigerant circuit and a compressor.

The invention relates to a linear compressor and a household refrigeration appliance of the type specified in the preambles of the independent claims.

WO 2008/055810 A1 and WO 2006/089582 A1 in each case disclose a linear compressor for a household appliance, with a piston movable in the longitudinal direction of a cylinder between an upper and lower dead-center position, whose piston outer surface can be stored in the radial direction of the cylinder in a contactless manner opposite a corresponding cylinder inner surface to form a gas pressure bearing. The linear compressors disclosed there are used in the respective refrigeration circuits of the household appliances.

Here, dependent upon the refrigeration circuit, gas is extracted out of the linear compressor from a low-pressure side of the refrigeration circuit and pumped into a high-pressure side of the refrigeration circuit. The gas pressure bearing used in the linear compressor is normally supplied from the high-pressure side of the refrigeration circuit, in order to prevent contact between the piston and the cylinder. When the linear compressor is actuated after an idle phase, the pressure at the high-pressure side and at the low-pressure side is customarily balanced. The gas pressure bearing can thus not be embodied accordingly and in the initial seconds after the start of the linear compressor the piston outer surface makes contact with the corresponding cylinder inner surface.

Over the service life of the linear compressor considerable wear and tear can thereby arise on the piston outer surface and on the corresponding cylinder inner surface. The wear particles thereby occurring can clog up corresponding gas pressure bearing nozzles, which with sufficient pressure on the high-pressure side store the piston opposite the cylinder in a contactless manner. The function of the gas pressure bearing can thereby be lost, and in the worst case a piston seizure can occur as a result.

WO 2006/069881 A1 likewise discloses a linear compressor for a household appliance, with a piston movable in a longitudinal direction of a cylinder between an upper and lower dead-center position, whose piston outer surface can be stored in the radial direction of the cylinder in a contactless manner opposite a corresponding cylinder inner surface to form a gas pressure bearing. According to the method of WO 2006/069881 A1, it is here provided for the mutually opposing lateral surfaces of the piston and of the cylinder to be provided with an abrasion-resistant anti-friction layer. This is however connected with significant additional expenditure and corresponding costs in the manufacture of the linear compressor.

It is the object of the present invention to provide a linear compressor and a household refrigeration appliance of the type cited in the introduction, by means of which the service life of a linear compressor can be extended and its functional effectiveness ensured in an improved manner.

This object is solved by means of a linear compressor and a household refrigeration appliance with the features of the independent claims. Advantageous embodiments with expedient and non-trivial developments of the invention are specified in the dependent claims.

The inventive linear compressor for a household appliance comprises a piston movable in the longitudinal direction of a cylinder between an upper and lower dead-center position, whose piston outer surface can be stored in the radial direction of the cylinder in a contactless manner opposite a corresponding cylinder inner surface to form a gas pressure bearing. For improved maintenance of the functional effectiveness of the linear compressor and to increase the service life of the linear compressor, according to the invention it is provided for the piston outer surface and the cylinder inner surface to have a martensitic microstructure at least in an area predefined in each case. In other words, the piston outer surface and the cylinder inner surface are embodied at least in the area predefined in each case in a martensitic steel.

Corresponding surface layers of the piston outer surface and the cylinder inner surface thereby exhibit no porous zones in the at least one area predefined in each case, instead of which a particularly wear-resistant and tough layer is provided on the surface of the areas predefined in each case. By means of the inventive solution an adequate resistance to abrasion on the surfaces of the areas predefined in each case can be achieved, so that the reliable formation of the gas pressure bearing can be guaranteed over the service life of the linear compressor. Furthermore, by means of the inventive solution abrasion of the piston outer surface and the cylinder inner surface can be very largely prevented, so that essentially no further wear particles occur. A possible clogging of corresponding gas pressure bearing nozzles, which serve to form the gas pressure bearing, is thereby securely prevented.

It is furthermore also possible to employ the inventive linear compressor in other applications than in household appliances, in particular anywhere that likewise self-sustaining gas pressure bearings or dynamic bearings, such as for example in turbines, are used. The inventive linear compressor can for example be used in corresponding devices of the aerospace industry and in energy generation plants.

In an advantageous embodiment of the invention it is provided that the piston outer surface and the cylinder inner surface in each case there comprise a single one of the predefined areas, where contact between the piston outer surface and the cylinder inner surface is possible if no gas pressure bearing is embodied or the gas pressure bearing has fallen below a predetermined pressure threshold value. The areas predefined in each case are thus only embodied with the martensitic microstructure where the danger actually exists that in the case of an insufficient supply of pressure for the linear compressor, contact between the piston outer surface and the cylinder inner surface is at all possible. It is thereby possible to save a laborious complete processing of the piston outer surface and the cylinder inner surface, which has a positive effect on the manufacturing costs of the respective components.

In a further advantageous embodiment of the invention it is provided for the cylinder to comprise a bushing, which comprises the cylinder inner surface. The correspondingly predefined area with the martensitic microstructure on the cylinder need only be embodied on the bushing, by means of which the manufacture of the cylinder can be significantly facilitated, as a corresponding surface treatment need be performed only on the bushing and not on the entire cylinder.

In a further advantageous embodiment of the invention it is provided that the respective predefined areas are embodied in steel with the alloy X15T.N. A particularly abrasion-resistant and low-wear surface is thereby provided on the areas predefined in each case.

In a further advantageous embodiment of the invention it is provided that the microstructures of the areas predefined in each case comprise homogeneously distributed, spherical carbides. A particularly high degree of abrasion-resistance of the areas predefined in each case is thereby achieved. The term spherical should be taken to means that the carbides are essentially embodied in spherical form, that is, within the usual manufacturing tolerances, have a maximum deviation from a theoretically optimal spherical shape in the range of about maximum 10 to 15%.

A further advantageous embodiment of the invention provides that the carbides have a diameter of 1 to 2 μm. A particularly fine microstructure in the form of ultra-finely distributed spherical carbides is thereby enabled, by means of which the abrasion-resistance of the predefined areas is further improved.

According to a further advantageous embodiment of the invention it is provided that the areas predefined in each case are heat-treated, surface-hardened and/or press-hardened. By means of an appropriate setting of respective process parameters and monitoring of these set process parameters, a corresponding surface layer can be embodied in the areas predefined in each case, which essentially exhibit no porous surface, by means of which the desired wear-resistant and tough layer can embodied on the respective surfaces. The corresponding microstructure can furthermore be embodied in a particularly simple manner by means of heat-treatment, surface hardening and/or press hardening.

The inventive household refrigeration appliance with a refrigeration circuit and a compressor is characterized in that the compressor is embodied as the inventive linear compressor or as an advantageous embodiment of the inventive linear compressor.

Further advantages, features and details of the invention are evident from the following description of preferred exemplary embodiments and on the basis of the drawings. The features and combinations of features previously cited in the description and the features and combinations of features cited below in the description of the figures and/or shown in the figures alone may be used not only in the combination specified in each case, but also in other combinations or in isolation, without departing from the scope of the invention.

Exemplary embodiments of the invention are explained in greater detail below on the basis the schematic drawings. Wherein:

FIG. 1 shows a schematic longitudinal sectional representation of an exemplary embodiment of a household refrigeration appliances with a linear compressor; and

FIG. 2 shows a sectional side view of the linear compressor, which comprises a piston movable in the longitudinal direction of a cylinder between an upper and lower dead-center position.

In the figures, the same or functionally similar elements are identified with the same reference characters.

Shown in FIG. 1 in a schematic longitudinal sectional representation is a household refrigeration appliance 10, which can be a refrigeration appliance, a freezer or a combination fridge-freezer. The household refrigeration appliance 10 comprises an interior compartment 12, which is embodied for the accommodation of foodstuffs. This interior compartment 12 can be a chiller compartment, a freezer compartment or a no-frost compartment or at least two of these compartments.

The household refrigeration appliance 10 comprises a refrigeration circuit 14, which comprises a linear compressor 16. The linear compressor 16 is preferably arranged in a machinery space 18, which is located in the lower rear area of the household refrigeration appliance 10.

The linear compressor 16 is employed in the refrigeration circuit 14, wherein gas is extracted out of the linear compressor 16 from a low-pressure side, which is not described in greater detail and pumped into a high-pressure side, which is not described in greater detail.

Based on FIG. 2, in which the linear compressor 16 is shown in sectional side view, this is explained in greater detail below. The linear compressor 16 comprises a piston 24 movable in the longitudinal direction 20 of a cylinder 22 between an upper and a lower dead-center position, whose piston outer surface 26 can be stored in the radial direction 28 of the cylinder 22 in a contactless manner opposite a corresponding cylinder inner surface 30 to form a gas pressure bearing, which is not described in greater detail.

In the present case the cylinder 22 comprises a bushing 32, by means of which the cylinder inner surface 30 is embodied. Via a multiplicity of drilled holes 34 extending through the bushing 32, pressurized gas can pass from a cavity 36 into a working chamber 38 and there embody the gas pressure bearing in the form of a gas cushion between the piston outer surface 26 and the cylinder inner surface 30, in order to store this in the radial direction 28 in a contactless manner.

The piston outer surface 26 and the cylinder inner surface 30 are embodied in an area 40, 42 predefined in each case by means of a martensitic microstructure. The predefined areas 40, 42 are here embodied where contact between the piston outer surface 26 and the cylinder inner surface 30 is possible, when no gas pressure bearing is embodied or the gas pressure bearing has fallen below a predetermined pressure threshold. This is in particular the case if the linear compressor 16 is actuated after an idle phase, wherein the pressure on the high-pressure side and on the low-pressure side is normally balanced. In such a case, the gas pressure bearing is not yet embodied or not yet fully embodied, that is with a correspondingly requisite pressure, so that the piston outer surface 26 and the cylinder inner surface 30 can come into contact with each other in the first seconds after the start of the linear compressor 16.

In the present case as shown, the predefined area 40 is embodied on those areas of the piston outer surface 26 which are arranged parallel to the cylinder inner surface 30. Furthermore in the present case the predefined area 42 is embodied on the entire cylinder inner surface 30 of the bushing 32. In alternative embodiments, the predefined areas 40, 42 can also be embodied only in part on the piston outer surface 26 and on the cylinder inner surface 30. For example the shape of the piston outer surface 26 and/or of the cylinder inner surface 30 can be such that these can touch each other only in certain sections. In such cases the predefined areas 40, 42 are also only embodied at these locations on the cylinder inner surface 30 and on the piston outer surface 26.

Undesirable wear on the piston outer surface 26 and on the cylinder inner surface 30 is prevented by means of the areas 40, 42 predefined in each case with their martensitic microstructure.

The areas 40, 42 predefined in each case are here embodied in steel with the alloy X15T.N. The microstructure of the areas 42, 40 predefined in each case here comprise homogeneously distributed, spherical carbides. The carbides have a diameter of 1 to 2 μm. The predefined areas 40, 42 can thereby be heat-treated, surface-hardened and/or press-hardened, in order to embody the correspondingly described martensitic microstructure with the spherical carbides.

Wear on the piston outer surface 26 and on the cylinder inner surface 30 over the service life of the linear compressor 16 is prevented or significantly reduced through the provision of the predefined areas 40, 42. It is thereby possible to prevent the occurrence of corresponding wear particles, which could clog up the drilled holes 34 serving as gas pressure bearing nozzles, so that adequate provision of pressure for the formation of the gas pressure bearing can be permanently ensured. In particular the danger of a so-called piston seizure can thereby be significantly reduced or even completely prevented.

LIST OF REFERENCE CHARACTERS

-   10 Household refrigeration appliance -   12 Interior compartment -   14 Refrigeration circuit -   16 Linear compressor -   18 Machinery space -   20 Longitudinal direction -   22 Cylinder -   24 Piston -   26 Piston outer surface -   28 Radial direction -   30 Cylinder inner surface -   32 Bushing -   34 Drilled hole -   36 Cavity -   38 Working chamber -   40 Predefined area -   42 Predefined area 

1-8. (canceled)
 9. A linear compressor for a household appliance, comprising: a cylinder having a cylinder inner surface and defining a longitudinal direction and a radial direction; a piston movably disposed in the longitudinal direction of said cylinder between upper and lower dead-center positions; said piston having an outer surface to be supported in the radial direction of said cylinder in a contactless manner opposite a corresponding cylinder inner surface by way of a gas pressure bearing; and said piston outer surface and said cylinder inner surface having a martensitic microstructure at least in predetermined regions thereof.
 10. The linear compressor according to claim 9, wherein each of said piston outer surface and said cylinder inner surface has a single said predefined region at a location where contact between said piston outer surface and said cylinder inner surface is possible, when no gas pressure bearing is embodied or when a pressure of said gas pressure bearing has fallen below a predetermined pressure threshold.
 11. The linear compressor according to claim 9, wherein said cylinder comprises a bushing forming said cylinder inner surface.
 12. The linear compressor according to claim 9, wherein said predetermined regions are formed of X15T.N steel.
 13. The linear compressor according to claim 9, wherein said microstructures of said predetermined regions in each case comprise homogeneously distributed, spherical carbides.
 14. The linear compressor according to claim 13, wherein said carbides have a diameter of 1 to 2 μm.
 15. The linear compressor according to claim 9, wherein said predetermined regions are heat-treated, surface-hardened and/or press-hardened.
 16. A household refrigeration appliance, comprising a refrigeration circuit and a compressor, said compressor being a linear compressor according to claim
 9. 